1. Field of the Invention
The present invention relates to apparatus for blow molding articles from molded preforms that have been reheated and are subsequently expanded within a blow mold to provide a desired hollow article. More particularly, the present invention relates to apparatus for heating portions of container preforms in a reheat blow molding machine, the apparatus including an auxiliary heating station at which auxiliary heating apparatus is provided for locally heating a longitudinally extending section of a preform to a higher temperature than the remainder of the preform.
2. Description of the Related Art
Blow molding machines are known for expanding a tubular plastic member that is positioned within a mold and subsequently expanded by pressurized air to provide a hollow article that has an outer shape that conforms with the interior surfaces of the mold. Such machines are sometimes single stage or extrusion-type, in which an extruder is provided for directly extruding a hot tubular plastic member, called a "parison," around which a pair of mold halves are closed, with the bottoms of the mold halves defining a "pinch-off." Pressurized air is then introduced through the top of the mold for expanding the parison to form the desired hollow article.
Another type of blow molding is accomplished by a two stage process, generally referred to as reheat-blow (RHB), in which plastic preforms are injection molded in a tubular form that is open at one end and closed at the other. After some time interval (during which cooling occurs), the preforms are received in a loading station, are conveyed to an oven for reheating to a softening temperature, and are then transferred to a blowing station at which the heated preform is positioned within a mold cavity and is subsequently blown by introducing pressurized air into the heated preform to provide the desired blown article.
Most often the tubular parison or preform is heated to a uniform temperature suitable for blowing. However, in instances when the molds define an article having one transverse axis longer than the other, such as a container having an oval cross section, uniform heating of the preform usually results in unequal distribution of the plastic material, with the areas of the preform that have been expanded radially outwardly the largest distance having thinner walls than those areas of the preform that have been expanded smaller distances.
One way to equalize more closely the plastic wall thickness distribution in a blown article is to compensate for the difference in radial expansion distance by making the tubular preform itself of non-uniform wall thickness. This approach has typically been used in single stage blow molding machines utilizing a thoroughly molten parison. In such cases it is fairly simple to compensate for an unsymmetrical configuration for the final product by means of extruding a parison initially which has a variation in thickness around the circumference. However, this approach is not well suited for two stage blow molding machines in which the preform is cooled before the final forming.
An alternate approach for achieving uniform wall thickness in unsymmetrical containers that has been used in two stage machines involves heating selected areas of the surface of the preform to different temperatures, thus permitting differential expansion of the respective portions of the preform. The areas of higher temperature will allow some material to be drawn away to help maintain wall thickness in the areas formed by the lower temperature segments that expand a greater distance. Although such localized heating of preforms is known, the previous approaches to providing that result were of limited effectiveness. In one method, the radiant heaters in an oven are specially configured to have the effect of producing a non-uniform temperature distribution in the preform. However, since these arrangements heat all areas of the preform at the same time (in the same oven), the arrangement of the heaters to produce defined areas of higher temperature consistently is very difficult to establish and is unique for each material selection and container configuration.
Another approach used in two stage blow molding machines involves first heating the entire preform in a conventional manner to a desired elevated temperature. Selective heating is then separately provided by masking or screening a portion of an auxiliary heating element to prevent an increase in temperature for a specific area(s) of the preform. However, this method is inefficient since the power supplied to the heater in the masked portion is wasted. The masking procedure is also labor intensive and is difficult to control since the radiation of heat to the masked areas cannot be completely avoided. Furthermore, this method will sometimes still require a preform having variable wall thickness to achieve satisfactory results in the final blown article.
Accordingly, it is an object of the present invention to overcome the deficiencies in the prior art arrangements, and to provide an improved auxiliary heating system for localized additional heating of blow molding preforms to enable differential expansion.